Method and means for treating gases



June 8, 1954 s. c. COLLINS 2,680,357

METHOD AND MEANS FOR TREATING GASES- Original Filed April l1, 1946 2 Sheets-Sheet l j N www3 NNNNN. r wm \N. 0 0 eC w NN. [J NNN| w m M .KNNQ

NNN NNN NN NNN June 8, 1954 s. c. COLLINS 2,680,357

METHOD AND MEANS FOR TREATING GASES Original Filed April l1, 1946 2 Sheets-Sheet 2 fr: 7 57@ 60 f @f7 s u MIR,

Patented June 8, 1954 METHOD ANI) MEANS FOR TREATING GASES Samuel C. Collins, Watertown, Mass., assignor, by mesne assignments, to Arthur D. Little, Inc., Cambridge, Mass., a corporation of Massachusetts Original application April 11, 1946, Serial No. 661,253. Divided and this application May 26, 1951, Serial No. 228,391

6 claims. (o1. ca -175.5)

This invention relates to improvements in a method and means for treating gases. It is herein particularly described in its application to the production of substantially pure oxygen from air but this is merely illustrative because the process and apparatus disclosed may be used with various gases to be processed such as air, blast furnace gas, water gas, coke oven gas and other manufactured gases, to produce especially desired products such as oxygen, nitrogen, argon, helium, hydrogen, carbondioxide and other gases. In some uses the desired product may be produced in the gaseous state and in other uses a liquid may be the end product.

It is among the objects of the invention to provide simple, compact and lightweight apparatus which can be permanently installed in some fixed location or on various vehicles such as aircraft, ships, railroad cars and trucks, or which can be mounted on a portable base and readily moved about to locations where its use may be more or less temporary. It is a feature of both the improved process and apparatus that in the treatment of a gas certain minor condensable components thereof are caused to be condensed' and thereby separated from the remainder by heat transfer so that later these separated components may either be recovered, if desired, or caused to be discharged so as not to prevent the attainmentA of the purity sought in the end product.

For example, when the apparatus is used in the production of a pure gas from impure gases, the latter are first compressed and subjected to thermal conditions which effect the depositions of certain undesired components during the initial ow of the impure gases. Later by a novel interchange of flow, these deposited components are removed and discharged with the waste gases resulting from the production of the desired pure gas. In another example, the gases to be processed may ow to a reaction chamber, such as a ractionating column or the like, wherein is produced the desired product and also waste gases which normally carry olf some of the desired product. These waste gases may be so heat treated as to cause the desired product contained therein to be separated from the remainder of the waste gases and deposited so that by a similar novel interchange of fiow the deposits will be picked up by the entering gases to be processed and returned to the reaction chamber. In one example the gases to be processed are cleaned and the undesired components taken from it are discharged with the waste gases and thereby precondensable components, the gaseous mixture is compressed and cooled to effect depositions of the condensable components during the initial flow of the compressed gaseous mixture. Later by a novel interchange of flow, a portion of the processed gas, at a substantially lower pressure, evaporates the deposits and consequently becomes enriched with the condensable components.

In the specic embodiment of the invention illustrated herein it is a feature of the invention that compressed air at a relatively low pressure is all passed in a single stream of flow through a heat exchanger, an expander, a second heat exchanger, the boiler of a fractionating column and thence through a third heat exchanger, all suitably connected in series, and finally admitted to the top of the column for purposes of rectication. It is also a feature that the waste products or eiiuent from the column are passed in reversed direction of flow through the third heat exchanger, the second heat exchanger and finally through the first heat exchanger. It is in the latter exchanger that the novel interchange of ows occurs whereby during one period the incoming compressed air moves through one pas-` sageway while the effluent is moving in a counter f direction of iiow through an adjacent passageway, and then during the next period the flows of the air and effluent are interchanged so that each fluid moves through the passageway in which the other fluid has just previously been flowing.

The provision of apparatus capable of producing substantially pure oxygen from low pressure compressed air effects a desired saving in the power expended in compressing air to the higherv pressures heretofore deemed essential, enables Y the apparatus to be much lighter in weight than of heat exchanger, a portion being in medial section, a portion in full side view, and other portions in full side view but with parts removed; and

Fig. 4 is a cross-sectional view of the heat exchanger taken as on line 4-11 of Fig. 3.

The apparatus disclosed will be described as it may be used for the production from atmospheric air of pure oxygen, either in the gaseous or liquid state. It is to be understood, however, that the invention contemplates the use of the invention in connection with other raw gases from which various desired products are to be obtained.

The separation of oxygen from air is accomplished by reducing the air to liquid form and distilling this liquid in a fractionating column. Since the normal boiling point of oxygen is about 90 K. (Kelvin) or about -29T F. (Fahrenheit) it is obvious that in the production of oxygen from air the problem is primarily one of refrigeration. Ordinarily the lower the temperature the greater the cost per unit of refrigeration. To efiiciently utilize refrigeration the apparatus of the present invention comprises three counteriiow heat exchangers A, B and C arranged in series with an expander D interposed between the rst and second exchangers and with a fractionating or rectifying column E interconnected with both the second and third exchangers. With these elements thus arranged in series the incoming air flows in a continuous stream to the top of the column and the waste gases from the column counterow through the three heat exchangers. Thus in each exchanger the heat possessed by the air is effectively transferred to the waste gases so that apparatus is used for making gaseous oxygen, this' gas is also passed through the second and nrst u heat exchangers to enhance the thermal effectiveness of these units.

For a more complete understanding of the performance of the apparatus it should be said that there are three distinct periods which occur from the starting of the apparatus to its oxygen producing operation. These are known as the cooling-down period, the liquefaction period and the distillation period. During the cooling-down period the temperatures of the various parts of the apparatus are reduced to their proper operating levels. During the liquefaction period a charge of liquid air sucient for rectification is produced. During the distillation period the desired substantially pure oxygen is continuously produced for discharge from the apparatus.

The flow of the air during the initial coolingdown period will now be described. When starting up compressed air at from 150 to 175 pounds per square inch (p. s. i.) and at ordinary room temperature is brought to an air inlet 2 of a valve casing 4 having two valve chambers E and 8 separated by a medial partition. The chamber 6 has an outlet I8 connected to a branch 12a of a pipe I2 that leads to an outer annular passageway I4 through the rst heat exchanger A. Chamber 6 has another outlet I6 connected by a branch [8a of a pipe I8 that leads to an inner annular passageway 28 of the same heat exchanger. The chamber 8 has an inlet 22 connected with pipe i8 and another inlet 24 connected by a branch |2b with pipe i2. The chamber 8 also has an outlet 25. In each chamber is a piston valve 28 and 38 respectively, secured to a common piston rod 32 which extends from the casing to a bell crank 34 pivotally mounted at 3B. When the valves 28 and 38 are at the left end of their respective chambers S and 8, the inlet 2 and the outlet I6 of chamber S are open while the outlet I0 is closed. Simultaneously in chamber the inlet 22 is closed while the inlet 24 and outlet 26 are open. When the valves are shifted to the right ends of the chambers, the inlet 2 and outlet l) of chamber 6 are open and outlet I6 is closed, while in chamber 8 the inlet 22 and outlet 26 are open and inlet 24 is closed.

Beyond the iirst heat exchanger is shown, in Fig. l, another valve casing 38 similar to the valve casing 4. This has two valve chambers 40 and 42 in which are piston valves le and l secured to a piston rod lia that connects with a bell crank '58 pivotally mounted at 52. Chamber il has an outlet 543 connected by a branch 56a and a pipe 5S with passageway lll of the heat exchanger A and has an outlet 58 connected by a pipe 68 with passageway 28 of this same heat exchanger. Chamber 42 has an inletV 62 connected by branch 88a of pipe es with the passageway 2G of the heat exchanger A and has another inlet 64 connected by branch 56h of pipe '5B with the passageway i4 of the exchanger. Chamber le also has an inlet EE and chamber i2 also has an outlet 68.

The bell crank 34 for the lower piston valves 28 and 3S and the bell crank 5G for the upper piston valves 44 and 48 are both connected by rods 'i8 and 'I2 with a suitable reversing mechanism 'M which is actuated by a shaft 'l having a worm and gear connection 'Vi with a shaft i8 of the expansion engine D. As the latter shaft turns continuously the reversing mechanism acts to translate this rotary motion into reciprocations of the rods 18 and 'i2 so that when the Valves 28 and 3i) oi' casing i are shifted from the left end to the right end of their strokes, the valves fili and of casing 38 will be shifted simultaneously from the right to the left ends of their stroke, and vice versa. The purpose and eiect of this periodic shifting of the several valves will be appreciated as the description develops.

With valves positioned as shown in full lines in Fig. 1, the compressed air iiows from inlet 2 through chamber Ei, outlet i6, branch ica and pipe i8 into passageway 28 of the heat exchanger A. From this passageway the air moves on through pipe 653 and branch 68a into valve chamber 42. From this chamber the air flows in a pipe 19 to a T 8B in a pipe 8l. This pipe 8l contains besides the T 88, a valve 82, Ts 88 and 84, another valve 85 and still another T 86 from which the pipe 8i continues to the second heat exchanger B. From the T BQ an auxiliary circuit, containing a valve 81a, extends through a pipe 8l', a coil 88 and a pipe 8S back to the T 33. The purpose and function of this auxiliary circuit will be explained later. For the moment let it be said that when starting up the valve 82 is Wideopen so that the air leaving the heat exchanger A passes directly through the T 80, the valve 82 and the T 83 to T 8d. Despite the fact that valve 85 is also open the air does not flow 5.. beyond T 84 in pipe 8| during the coolingldown period, but flows through a pipe 90 to a surge chamber 9| which is connected by a pipe 92 with the inlet 93 of the expansion engine D. The details of a preferred form of expansion engine are shown in a copending application Serial No. 665,206, filed by me on April 26, 1946. Su'ice it to say here that suitable valves and mechanism are provided for admitting the air to the expander, permitting it to do worktherein and then, at greatly reduced temperature and presf sure, exhausting the air from the engine outlet 94 through a pipe 96 to another T 98 in a pipe |00.

When the apparatus is producing oxygen, that is during the distillation period, all the air from the expander D reaching the T 98 ows thence through a second tank |02 into the second heat exchanger B, from which it goes into a boiler I 04 in the bottom of the fractionating column E, where it is liquefied, and thence moves onward through the third heat exchanger C to be expanded by a suitable device |06 (either in the form of an expansion valve, as indicated in the drawings, or a suitable length of capillary tubing) and eventually is admitted into the top of the column E. During the cooling-down period, however, the air flow isdiiferent because the expansion valve |06 is then kept closed. This prevents any low of air from the T 98 in the manner just described for the distillation period.

During the cooling-down period the air reaching the T 98 moves onward through the pipe |00, past an open valve |08 therein, to a T vI I0 in a pipe ||2 which extends between an outer annular passageway |I4 of the second heat exchanger B and a like outer annular passageway |I of the third heat exchanger C. This latter passageway is connected by a pipe |48 with the top of the column E for flow of eluent or waste gases from the column during the distillation period. But since the expansion valve |06 is kept closed during the cooling-down period no air reaches the top of the column and no rectiiication occurs in the column during this period. Hence there is no flow of eiliuent from the column through exchanger C and pipe ||2 to the T |I0. Indeed, because of this closure of the expansion valve |06 no flow whatever can occur in pipe I|2 in the direction toward the third exchanger C. As a result the air reaching T I |0 flows in the other direction through pipe ||2 into the annular passageway II4 of the second heat exchanger B and thence moves along a pipe I I6 to the valve chamber 49. The air leaves this chamber through outlet 54 and ows along branch 56a and pipe 56 into the outer annular passageway I4 of the rst heat exchanger A. From the latter the air goes through pipe I2, branch |2b and the inlet 24 into the valve chamber 8 and is then discharged from the apparatus through the outlet 26.

Thus during the cooling-down period the air follows a limited path of flow through the first heat exchanger A, the expander D, the second heat exchanger B and back through the rst heat exchanger A again. Its second passage through the first exchanger A is in a direction counter to that of its rst flow through the unit. As before noted when starting up the air enters the rst heat exchanger at about room temperature and at a pressure of from 150 to 175 p. s. i.

When the air enters the expansion engine it is expanded and in so doing transfers its energy to the piston of the engine. This not only re- 6.. duces the pressure of the air but substantially lowersvits temperature. Since this cooler air leaving the expander owsback through the first heat exchanger the incoming air is cooled by the loss of heat to the colder outgoing air. This progressive cooling by heat transfer and "by the energy given up by the air in the expander continues for about an hour or an hour and a quarter. By then the air leaving the expander has reached a temperature of about 103 K. or 274 F., and when this occurs the cooling-down period may be deemed at an end. Not only is the air leaves the expander at a very low temperature but all the apparatus will have been cooled down to a point whereat the liquefaction period can be initiated.

This is done 'by cracking open the expansion valve |06 so that about 6% of the air will flow to the column E. This small quantity of air is not taken from the low pressure air leaving the expander but is taken from the high pressure air at T 84 ahead of the expander. This air flows from this T 84 through the pipe 8| which leads to an inner annular passageway |20of the second heat exchanger B. In this pipe 8| are the open valve and the T 86, the latter being connected by a pipe |26 with the second surge tank |02. A check valve |28 in this pipe |26 opens for flow from the surge tank but closes against flow in the opposite direction. The high pressure of the small quantity of air flowing in pipe 8| beyond the T 84 keeps this check valve |28 closed against any tendency of flow of the air at lower pressure leaving the expander and reaching the T 98. As a consequence the small quantity of high pressure air flows in one direction through the second heat exchanger while the low pressure air, which has passed through the expander and has been thereby cooled, flows in the opposite direction through the same exchanger. The transfer of heat from the high pressure air to the low pressure and colder air results in the liquefaction of the high pressure air which moves on through a pipe |30 to an air inlet manifold |32 at the bottom of the fractionating column E. From this manifold the air flows through a coil of tubes |33 in the boiler and then moves on through a pipe |34 to an inner annular passageway |36 through the third heat exchanger C, and thence along a pipe |33 to the expansion valve |05previously referred to. This is the valve that is cracked open to initiate the lique- 'action period. 'I'his slight opening permits the air to flow along a pipe |40, past Ts |42 and |44 and an open valve |46, to enter the top of the column E wherein it trickles downward and collects in the space of the boiler around the coil of tubes therein.

As the level of the liquid air collecting in the boiler rises its changing level is indicated by a U-tube |48 containing a suitable indicating liquid and having connection with the lower partof the boiler through a pipe |50, and with the space above the boiler proper throughanother pipe |52. It takes from about three quarters of an hour to an hour to produce this desired quantity of liquid air in the bottom of the column before the rectification is begun.

The rectification is initiated by opening the expansion valve |06 somewhat wider. If the product is to be low pressure gaseous oxygen the valve 85 in pipe 8| and the valve |08 in pipe |00 are closed. This causes all of the air to pass from the T 84 through the surge tank 9|, the expander D, and through the surge tank |02. It

flows past the check valve |28 to T 86 and thence through pipe 8| into the inner annular passageway |28 of the second heat exchanger B. Thence it ilows through pipe |30 into the coil of tubes in the boiler. The setting of the expansion valve |08 should be such as to establish a pressure of about 70 p. s. i. in the boiler coil. Under this pressure the air in the coil readily condenses and becomes wholly liquid, the heat given up during the condensation passing to the liquid air that has collected about the coil in the boiler.

The liquid air in the coil moves on through the third heat exchanger C' to the expansion valve |08 and thence into the top of the column E whence it trickles down through and over suitable packing therein. The details of a preferred form of column are disclosed in a copending application, Serial No. 674,521 filed June 5, 1948 by Howard O. McMahon. During this downward iiow of the air and the upward flow of the vapors from the boiler the process of rectication takes place so that by the time the down-flowing liquid reaches the boiler it is substantially pure oxygen and by the time the vapors reach the top of the column they consist largely of nitrogen, some argon and a slight quantity of oxygen. All this constitutes the so-called eftluent which flows through pipe l I8 into the outer annular passageway of the third heat exchanger C. It flows therethrough in a direction counter to that of the liquid air owing in the inner annular passageway |36.

The temperature of the effluent entering the third heat exchanger is about 86 K. or approximately 305 F. The temperature of the liquid air leaving the coil in the boiler and entering the third heat exchanger is about 98 K. or 288 F. Accordingly the liquid air gives up heat to the eliiuent as they both flow through the third heat exchanger and the air reaches the expansion valve |08 with a temperature of about 92 K. or 294 F.

Vrlhe eilluent moves from exchanger C through pipe H2 to one end of exchanger B for flow through the outer annular passageway |18, being then at a temperature approximating 96 K. or 285 F. The gaseous air from the expander D has a temperature of about 110 K. or 261 F. when it enters the other end of the second heat exchanger B. Consequently during its flow through this exchanger the air gives up heat to the effluent and reaches the coil in the boiler at a temperature of about 100 K. or 279 F. rThe effluent or waste gases ilowing on through pipe I8 and the valve chamber 40 reache the first heat exchanger at a temperature in the neighborhood of 115 K. or 253 F. Since the compressed air comes to this exchanger at a temperature of say from 300 K. to 322 K., or from 80 F. to 120 F., it gives up heat to the counter-howing waste gases and is gradually cooled. to about 135 K. or 234 F. for admission to the expander D. The waste gases finally leave the apparatus through the outlet 28 at close to room temperature. l

Aiding in the cooling effect in the rst and second heat exchangers A and B is the provision for the oxygen gas to flow through these exchangers. A pipe |54 upstanding in the boiler above the level of the liquid oxygen therein is the outlet for the substantially pure oxygen gas. This pipe extends through the manifold |32 and leads to a T |58 from which a pipe |58 connects with a central passageway |60 through the second heat exchanger B. From the latter a pipe |62 leads to a similar central passageway |84 through the first heat exchanger A. From the latter the oxygen gas leaves through a pipe |68 having a valve |68 controlling the delivery of the gaseous oxygen from the apparatus through the outlet |10.

During normal running of the apparatus the pressure of the compressed air entering the inlet 2 may be somewhat lower than is preferred during the cooling down period. A satisfactory operating range is from 120 to 150 p. s. i. and under some conditions the compressed air may have a pressure as low as 65 p. s. i. If 100 cubic feet of compressed air is supplied at about room temperature and within the range of pressure noted, then from A10 to 20 cubic feet of substantially pure oxygen gas 99.6% O2 will be delivered at approximately room temperature with a pressure of about 5 p. s. i. This process of distillation may continue indefinitely. In the design of the apparatus operating pressures are so chosen as to produce at all times more refrigeration than is needed. If the level ofy liquid oxygen in the boiler tends to climb too high as a result of excessive refrigeration valve 85 in pipe 8| is opened slightly so that a fraction of the high pressure air ley-passes the expander D. The total refrigeration effect decreases accordingly.

If high pressure oxygen gas is the product desired, a valve |12 in pipe |511 is closed and liquid oxygen is drawn from the boiler through a pipe il'fl which has a T ile and a valve |l8 therein controlling a liquid oxygen outlet E88. Valve |18 is normally closed and so the liquid oxygen iiows from T |18 through a Valve 82 to a suitable pump F capable of producing any desired pressure up to say 3000 p. s. i. The liquid oxygen leaves the boiler at about K. or 288 F. and leaves the pump at about 85 K. or 806 F. The pump may be driven by the expansion engine D or by any other source of power. It is maintained at a temperature oi about 85 K. or 306 F. by a jacket or coil |84 through which the expanded liquid air is passed. This is accomplished by closing valve |48 in pipe |40 and opening valve |88 in a pipe |90 leading to the coil |84. From the latter the expanded air flows back to the T |84 through a pipe |82.

From the pump F the high pressure liquid oxygen ows through a pipe |84 to a T |08 and thence along a pipe |98 past an open valve 200 to the T |56. From this T the high pressure liquid oxygen ows through the second and rst heat exchangers along the path of how previously described for the low pressure oxygen gas. During its travel through the exchangers the liquid oxygen absorbs suicient heat, especially in heat exchanger A, to convert it to gaseous oxygen which is delivered at the outlet |10 at the high pressure desired.

If only low pressure liquid oxygen is desired, valves |l'2 and |82 are closed and the liquid oxygen is drawn directly from the column through the pipe |14 and discharged through the outlet |80. Such liquid product will be at about 7 p. s. i. and a temperature close to 95 K. or 283 F. If the liquid oxygen is to be delivered at high pressure, valves |12, H8 and 200 are closed and valve |82 opened to admit the liquid oxygen to the pump F. It is increased to the desired pressure and delivered through pipe 04 to the T |96 whence it passes to a valve 282 and is discharged at an outlet 204.

With the several valves set as just described the yield of liquid oxygen or high pressure oxygen gas will be much less on a weight basis than the yield of low pressure gaseous oxygen because of the greater refrigeration requirements of these products. However, this yield can be substantially increased if the conditions of operation characteristic of the liquefaction period are approximated by opening wide the valves and |08 and adjusting the expansion valve |06 to permit enough liquid air to enter the column E to secure the required purity of product.

In 1Fig. 2 there is disclosed a modication in the means beyond the heat exchanger A for controlling the flow of the air and the effluent. The modification consists in replacing the mechanically actuated valves 44 and 46 oi Fig. l by a series of check valves so arranged that as the flow through the heat exchanger is altered by thel operation of the valves 23 and 30 the pressure of the fluids themselves will determinel their proper course to the T 84 and from the pipe II6.

Assuming that the mechanically operated valves 23 and 30 are in the positions shown in full lines, the air will iiow from inlet 2, through the valve chamber 6 and thence through branch Ia and pipe I8 into the inner annular passageway 20 through the heat exchanger A. This passageway is connected by a pipe 206 with a T 208 from which a short pipe 2I0, containing a check valve 2I2, leads to another T 2I4 to which the pipe II6 is connected. From the T 208 a pipe 2I6 leads to still another T 2I8 from which a pipe 220 runs to the T 86. T 2I4 is connected with T 2 I8 by a pipe 222 containing a check valve 224, a T 226 and another check valve 230. From the T 226 a third pipe 232 leads to the outer annular passageway I4 to heat exchanger A.

Air entering pipe 206 under relatively high pressures, as compared with that of the efliuent, cannot pass on to pipe II6 because of the check valve 2I2 in pipe 2I0 between the Ts 203 and 2 I4. This check valve 2 I2 opens for flow toward the heat exchanger and closes upon a tendency to ilow in the reverse direction. In the pipe 2 I6 is another check valve 234 which opens for flow toward the expander D or away from the T 208 but closes upon any ow toward this T or toward the heat exchanger A. In the cross pipe 222 the two check valves 224 and 230 open for iiow in the direction from T 2I4 toward T Zl and close against flow in the reverse direction.

As the air enters pipes 206 and 2I0 it effects closure of the check valve 2 I2 thereby preventing any flow of air onward and resisting any flow of the eiiiuent from pipe II6. The air opens the check valve 234 and flows on through pipe 2I6 and pipe 220 to the T Stand thence to the T 84. The pressure of this air in the end of the cross pipe 222 next to T 2I8 holds check valve 230 closed. This stops both flow of air and now of eiiiuent past this valve.l The eluent coming through pipe I I6 cannot flow beyond check valve 2I2 in pipe 2I0, as already noted, but can open check valve 224 in the cross pipe 222 and ilow thence through the T 226 and pipe 232 to the annular passageway I4 of the heat exchanger. Also as previously noted, the eiiluent cannot get past check valve 230 which is held seated bythe higher air pressure upon it from pipe 2 I6.

Upon the shifting of the valves 28 and 30 to the right ends of chambers 6 and 8 air will flow from the inlet 2 through the chamber I5 into branch I2a and thence through pipe I2 into the outer annular passageway I4 of the heat exchanger A. It will then pass upward in the pipe 232 to the T 226 and into cross pipe 222. Its pressure will seat the check valve 224 and hold it closed against the lower pressure of the eiuent from pipe II6 but will open the check valve 230 to permit the air to flow to T 2I8 and through pipe 220 to the T 80. The air pressure .thus effective in the pipe 2 I6 will keep the check valve 234 seated. The eiuent coming through pipe II6 and into pipe 2I0 will open and flow past the check valve 2 I2 into pipe 2%` and thence to the inner annular passageway 20 through the heat exchanger A. The eiiiuent cannot'pass beyond check valves 224 and 234 because both are held closed by the air pressure on their opposite sides.

Thus in the modied arrangement shown in Fig. 2, the sarne interchange of ilow of gases to be processed and waste gases occurs in the two annular passageways I4 and 20 through the heat exchanger A, and the pure gas always flows in the pipe |62 and through the central passageway I64 through the exchanger A.

The importance of this interchange of flow is appreciated when it is realized that as the compressed air passes through the rst heat exchanger A certain of its components are condensed and deposited on the internal surfaces of the passageway through which the air is moving. This occurs because the eiuent or waste gases absorb so much heat from the air owing in the opposite direction in the closely adjacent annular passageway that the components of substantially higher boiling point are reduced in temperature below the condensation point. The water vapor in the air is the iirst such component to be condensed and a deposit thereof soon begins to collect in the passageway. Somewhat beyond, the temperature conditions are such that the carbon dioxide in the air will likewise condense and begin to gather on the surfaces in the passageway. These deposits would in time clog the passageway and reduce the flow of the air to a prohibitive extent and iinally shut it ofi altogether. The clogging is avoided by the present invention.

During the flow of the waste gases through the passageway in which the air has been owing and in which the deposits have accumulated, the waste gases evaporate, sublimate or otherwise remove the deposited components and carry them off through the outlet 26, thus cleaning the passageway and preparing it for the next shifting of the valves and interchange of flows. Accordingly not only does the periodic interchange of ows prevent the clogging or plugging of the passageways but in the particular use of the invention in connection with the production of oxygen, the purity of the latter is from the outset made more readily attainable since some of the undesired components of the air are removed therefrom during the initial iiow of the air through the first heat exchanger. Thus the apparatus is especially eflicient in supplying oxygen, either in the gaseous or liquid state, in a substantially pure condition.

The duration of the periods between interchange of flows must be such that substantially valves in response to a gas volume meter through which is passed either the gas to be processed or the waste gases. By this means the flows are interchanged periodically, the duration of each period being determined by the volume of gas which has passed through the meter during the period. The expansion engine herein described is a special kind of gas volume meter and accordingly it may conveniently be used to actuate the switching valves in the manner already hereinbefore indicated.

In order to remove the deposits collected during any period by the returning gases during the following period, it is necessary to adjust conditions of temperature and pressure so that the evaporative capacity of the returning stream is greater than that of the entering stream. The evaporative capacity of a certain mass of gas is determined b'y the pressure and temperature oi' the gas, or alternatively by its volume and temperature, or again alternatively by its pressure and volume. In other words, any two of the three conditions of pressure, temperature and volume are suicient to determine the evaporative capacity of a certain mass of gas. For the Vsake of clearness the conditions of pressure and temperature will be discussed.

For a given mass at a given temperature the evaporative capacity is inversely proportional to the pressure. In other words, as the pressure is decreased the evaporative capacity for a condensed component is increased proportionately. Thus, if a mass of compressed gas is cooled until condensable components begin to separate out, and ii the pressure is then decreased keeping the temperature constant, the condensable components will evaporate. In order for the clean-up system as herein described to function properly the pressures of the gas to be processed and the waste gases must be such as to insure complete removal of the condensed deposits.

In some cases it may be necessary to add or to subtract heat at one or more locations along the heat exchanger in order to provide suitable temperature conditions for the complete removal of the deposits. The evaporative capacity of a certain mass of gas at constant pressure depends upon the temperature in two distinct ways. As the temperature is increased the vvolume increases in direct proportion to the absolute temperature; hence the evaporative capacity increases correspondingly. superimposed upon this eiiect is the condition that the vapor pressure lof the condensed deposit also increases with temperature, generally according tothe approximate mathematical law where C and b are constants characteristic of the deposit and where T is the absolute temperature and lo'g p is the logarithm of the vapor pressure of the deposit. The overall effect of changing the temperature of a system comprised of a certain mass of gas in equilibrium with a condensed deposit is to increase the evaporative capacity of the gas much more rapidly than the rst power of the temperature.

In some instances it may occur that the conditions of pressure and temperature which naturally arise in -a heat exchanger are not favorable for the complete removal `of the deposits. `In such cases pressures or temperature must be articially adjusted to meet the conditions specified in the kforegoing. Thus it `Imay be necessary to `introduce 12 or withdraw heat artiiicially at certain regions along the length of the heat exchanger.

Thus in the production of oxygen from atmospheric air, if the air pressure is not suihciently high, substantial deposits of carbon dioxide may be formed along such a considerable length of the heat exchanger that the evaporative capacity of the returning waste gases is not suicient to remove the deposits completely.

One way of overcoming this disability is to compress the air to a higher pressure so that the deposits oi carbon dioxide form at a higher temperature. Another way of overcoming the disability is to recirculate at least a portion of the compressed air over that region of the heat exchanger at which the carbon dioxide deposits occur, after the compressed air has emerged from the heat exchanger and before it has gone to the expansion engine. This arrangement results in artiiicially cooling the heat exchanger at the region where carbon dioxide is normally deposited and extends the deposit region towards the warm end of the exchanger where the evaporative capacity of the returning Waste gases is sufficient to remove the deposits.

This latter arrangement is shown in Figs. 1 and 2 where the pipe 8l, containing valve Bla, leads from the T 83 to a coil 8S wrapped around the upper or colder end of the heat exchanger A. From this coil the pipe 89 leads to the T S3 in the pipe el. By a suitable setting of the valve 2 part of the compressed air owing from the heat exchanger A will pass through the pipe 3l, coil 98 and pipe 8e to the T 83 whence it joins with the air passing through valve 32 on its way to T Se and the expander D. The compressed air iiowing in coil 88 cools this region of the heat exchanger and because of this the carbon dioxide is deposited on the walls of the exchanger at a point nearer Vthe end of the exchanger where the relative warm lcompressed air enters it. By thus causing the deposits of carbon dioxide to occur ahead of where they would accumulate if the coil 83 were not employed, the deposits are more readily taken up by the waste gases since their evaporative capacity becomes greater as they proceed through the heat exchanger. If under certain conditions the function of coil 83 is not deemed necessary, it can be rendered inactive by closing the valve Bia.

In Figs. 3 and 4 is 'shown a preferred form of heat exchanger which can be used for any or all of the heat exchangers A, B and C shown diagrammatically in Figs. l and 2. This heat exchanger comprises three thin walled tubes 238, Zet and 2l2 and two coils 26d and and the necessary solder to bind the coils and tubes together. IThe innermost tube 238 constitutes the passageway fc4 for the pure Aoxygen or whatever pure gas is being produced. Around this is wrapped the coil lii which is formed by helically wrapping on edge a thin, flat, narrow strip ci good heat conducting metal about an imaginary axis to form a sort of .helical spring, and then helically winding this spring about the tube 2358 as shown. At the same time a wire of solder 26,8 is laid between the convolutions of the coil next to the tube 238. With the ends of tube 238 plugged, the tube and assembled coil 2&4 are immersed in a solder bath for bonding the coil `to the tube. Next the tube 2li-ii is slipped over the coil 244 and then drawn through a die to shrink it into tight contact with the outer edge portions of the individual turns of the flat strip which 13 makes up the coil 244 between the tubes 228 and 246 is the inner annular passageway 20.

Another coil 246 of somewhat narrower thin flat metal is first helically wound on edge into spring-like shape and then helically wrapped about the intermediate tube 24|), with a wire of solder 248 likewise interwound with it on the tube. With the ends of tube 240 also plugged the thus partly assembled exchanger is dipped into a solder bath to bond coil 246 to tube 240. During this immersion in the solder bath the solder in the annular passageway between tubes 238 and 240 remelts and bonds the coil 244 to both of these tubes. Then the outer tube 242 is slipped over the coil 246 and drawn through a die to shrink it tightly into contact with the coil 246. Thus is provided an outer annular passageway i4 through the coil 246 between thetubes 246 and 242. The entire assembly is then heated above the melting point of the solder to insure that both coils are tightly bonded to the tubes which they contact.

This manner of making up the heat exchanger insures that all the adjacent metal parts are in good heat-conducting contact with one another, and since the tubes 238, 240 and 242 have thin walls, the gases may be said to be in good thermal relation to one another. By following the proportions shown in the drawing (Figs. 3 and 4) the surface exposed to gas contact and the resistance to flow of gas is approximately equal in the inner annular passageway 20 and the outer annular passageway I4.

The amount of surface provided in the annular passageways and the relative weights of the gases to be processed and the waste gases flowing per unit of time dictate the proper length for the passageways to give the required efficiency of performance to produce the desired pure gas. The flow of the latter through the innermost passageway 164 aids in the cleaning performance because it cooperates in the exchange of heat and desirably lessens what would otherwise be a greater difference in temperature between the gases to be processed and the waste gases. By making the exchanger great in length relative to the diameter, the end where the raw gases are introduced may be at a temperature widely different from that of the other end and there will be no excessive leakage of heat along the exchanger. The heat exchange is effectively made between the gases flowing in the exchanger itself.

Although a heat exchanger as shown in Figs. 3 4 has proved highly eicient, it is to be understood that various embodiments of an exchanger can be used in the apparatus of Fig. 1. It is bebelieved, however, that to obtain the best thermal performance the principles of construction underlying the preferred form of Figs. 3 and 4 should be followed, namely having the gases separated so far as possible by only a single wall having high heat conductivity, and in both iiow passageways between which the interchange of ow is made having an extended metal surface packing in good thermal contact with the wall of the passageway. A heat exchanger constructed in accordance with these principles will perform the two important functions essential to the desired operation, namely, 1) a most eiiicient exchange of heat to insure the deposition of some of the components on the internal surfaces of a passageway through the exchanger and (2) the prompt and complete removal of these deposits so that the apparatus as a whole may continue to perform without loss of its effectiveness.

The present application is a division of my co-pending application, Serial No. 661,253, filed April 11, 1946.

I claim:

1. The method of rectifying a gaseous mixture which comprises dividing a main stream of the compressed gaseous mixture into a first stream and a second stream, expanding said first stream with the performance of external work, passing the expanded first stream successively into heat exchange relation with said second stream and said main stream, continuing the operation for a period sufficient to condense said second stream into a liquefied mixture, passing said liquefied mixture into a rectifier, arresting the flow of said second stream in response to a predetermined accumulation of the liquefied mixture in said rectifier, and thereafter passing the rectified products into heat exchange relation with the expanded first stream and said main stream.

2. The method of rectifying a gaseous mixture which comprises dividing a main stream of the compressed gaseous mixture which comprises dividing a main stream of the compressed gaseous mixtuer into a iirst stream and a second stream, expanding said first stream with the performance of external work, passingthe expanded iirst stream successively into heat exchange relation with said second stream and said main stream, continuing the operation for a period sufficient to condense said second stream into a liquefied mixture, passing said liquefied mixture into a rectier, shifting the course of said expanded first stream when a predetermined amount of liquid has accumulated in said rectiiier, so as to cause the pressure and temperature of said expanded first stream to rise and arresting the iiow of said second stream, and thereafter passing the rectified products into heat exchange relation with the expanded iirst stream and said main stream. y

3. The method of rectifying a gaseous mixture which comprises dividing a main stream of the compressed gaseous mixture into a first stream and a second stream, expanding the rst stream with the performance of external work, conducting said second stream into an elongate passageway having an outlet leading to a rectifier, passing the expanded first stream successively into heat exchange relation with said second stream and said main stream, continuing the operation to condense said second stream in said passageway to a liquefied mixture, passing the liquefied mixture into said rectier, accumulating a predetermined quantity of said liquefied mixture in said rectifier, arresting the flow of said second stream, and thereafter passing said first stream of expanded gas intoV indirect heat exchange relation with the liquid accumulating in said rectifier and a rectified product.

4. A method of starting up a process for liquefying and rectifying mixed gases, which comprises cooling a compressed stream of mixed gases, expanding the cooled compressed stream, passing the resulting expanded stream successively through a ow path in a heat exchange Zone to cool said zone and in heat exchange relation with said compressed stream; thereafter, while continuing the foregoing steps, dividing from said compressed stream a by-pass stream, passing said by-pass stream through a second flow path in said heat-exchange zone and therein liquefying said by-pass stream, passing the resulting liqueed by-pass stream into a rectifying zone; and thereafter arresting the flow of said by-pass stream to said second flow path and passing said expanded stream into said rectifying zone, While passing a product of rectification successively in heat exchange relation with said expanded stream and Said Compressed stream,

5. The method of claim 4, in which the now of said icy-pass stream is arrested in response to a predetermined accumulation of liquid in said rectifying zone and simultaneously the course of said expanded stream is shifted from the first mentioned OW path to said second new path while said product of rectification is passed through the rst mentioned flow path.

6. The method of starting up a proeess for rectifying mixed gases, which comprises cooling a compressed stream of mixed gases, expanding all of the cooled compressed stream, passing the resulting expanded stream successively through a dow path in a heat exchange zone to cool said zone and in heat exchange with said compressed stream to cooi said `compressed stream; thereafter, as soon as said heat exchange zone is suciently cool to liquefy said compressed stream and while continuing the foregoing steps, dividing a by-pass stream from said compressed stream, passing said oy-pass stream through a second 'ow path in said heat exchange zone in countercurrent heat exchange relation with said expanded stream, liquefying said ley-pass stream While in transit through said heat exchange zone, and Without substantial liquid holdup in said heat exchange zone passing the liquefied by-pass stream into a rectifying zone to cool said rectitying zene and accumulate therein a body of liquid; and thereafter carrying out the rectication of the liquid in said rectifying lzone.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,951,183 De Baufre Mar. 13, 1934 2,239,833 De Baufre Apr. 29, 194i 

